Sensing devices for monitoring strain in structural applications are well known. For example, it is known to incorporate strain sensors and indicators in structures, such as bridges, pipes and aircraft, as part of a means for predicting fatigue or identifying areas which have become damaged. Such sensors, however, are typically difficult to install, prone to damage, or require electricity. These difficulties are exacerbated when they cannot be incorporated during initial construction, (e.g. when placing them on a fifty year old bridge). Additionally, strain sensors are difficult to apply around elongated structures for measuring strain radially.
Conventional prior art strain sensors for sensing or indicating strain in an underlying structure are varied. Fatigue fuses, for example, are thin pieces of metal which are designed to break at a given percent of the underlying structure's fatigue life. It is also known to use fiber Bragg gratings (FBG) embedded in a structure where strain is to be measured or embedded in fiber-reinforced plastic and applied using an adhesive or welding. Strain gauges based on changes in electrical resistance, and which can be glued to the structure to be measured, are also well known.
In a seemingly unrelated field, monitoring strain in cable tie applications is also of importance. A cable tie or tie-wrap, also known as a hose tie, zap-strap or zip tie, is a type of fastener for holding items together, such as electric cables or wires. Cable ties have been used in cable harnesses and bundling, packaging, labeling and locking.
The common tie-wrap, normally made of nylon, has a tape section with teeth that engages with a pawl in the head to form a ratchet so that, as the free end of the tape section is pulled, the tie-wrap tightens and does not come undone. Another version of the tie-wrap allows a tab to be depressed to either adjust the tension or remove the tie-wrap. Another popular design of the cable tie locking mechanism involves a metallic barb in the head slot.
A cable tie has two typical failure mechanisms. First, the cable tie strap can break when the fastening load is beyond the material strength limit. Another failure mechanism involves the unlocking of the strap from the cable tie head slot, whether the cable tie uses a plastic pawl or a metallic barb. Both failure mechanisms can cause a cable tie to fail before reaching its designated loop tensile strength rating. In many uses, the cable tie endures continuous or intermittent strain and its fatigue may pose safety threats.
Accordingly, it would be desirable to provide a means for large scale, cost efficient, widespread strain monitoring. Because of their low cost and ease of use, tie-wraps are ubiquitous, finding use in a wide range of other applications, and can, therefore, meet this need. In particular, it would be desirable to provide a strain sensing or fatigue indicating cable tie for use in both structural and cable tie applications.
A separate problem exists when installing cable ties with respect to pretension. Proper installation requires a small amount of tension, which is rarely accurately measured, leading to inconsistently applied cable ties. In large installations this may mean that some cable ties bear more load than others, resulting in premature and uneven failure.
To solve this problem, the prior art has developed cable tie tension tools, which tighten cable ties to a user-calibrated, predetermined tension limit. However, such tools are difficult and expensive to use.
It is therefore desirable to integrate some indicator of tension into the cable tie itself in order to apply the same amount of pretension to each cable tie in an installation